In industrial applications surface contours typically often have to be tested for dimensional compliance. Thus, optical touch free scanning methods are often used in order to prevent surfaces from getting damaged.
A widely used method is the so called light section triangulation method where a line of light is radiated onto the surface by the laser and the laser light reflected by the surface is captured by an optical sensor like e.g. a CCD sensor from a viewing direction that differs from the impact direction of the laser.
From a shape and extension of an image of a laser light line on the optical sensor an actual contour of a work piece surface at a location of the light radiated onto the surface can be computed in view of an angular offset between the applied laser light fan and the viewing direction of the CCD sensor.
The entire surface or a surface portion can be scanned and measured in this manner by a relative movement of the laser light line transversal to its extension.
In order to be able to measure recesses, undercuts etc. it is best when the laser light fan is arranged in a plane that is perpendicular to the surface to be scanned.
However, this perpendicular direction of irradiation has its problems when                the surface either has a strong mirror effect because the laser light is then reflected again in the same plane, or        the surface is arranged on a transparent material since the laser light then penetrates the transparent surface and the material below without being reflected at all.        
The problem can only be partially circumvented in that the laser light fan is irradiated in a plane that is not orthogonal to the surface since the processing as recited supra can be more difficult or the contour determination can have errors.